The present invention relates to an improved steam turbine. More specifically, the present invention relates to improvements in the stationary parts of a steam turbine, specifically, the cylinder and the inner flow guide of the exhaust diffuser.
The steam flow path of a steam turbine is formed by a stationary inner cylinder and a rotor. A large number of stationary vanes are attached to the inner cylinder in a circumferential array and extend inward into the steam flow path. Similarly, a large number of rotating blades are attached to the rotor in a circumferential array and extend outward into the steam flow path. The stationary vanes and rotating blades are arranged in alternating rows so that a row of vanes and the immediately downstream row of blades form a stage. The vanes serve to direct the flow of steam so that it enters the downstream row of blades at the correct angle. The blade airfoils extract energy from the steam, thereby developing the power necessary to drive the rotor and the load attached to it.
As the steam flows through the turbine its pressure drops through each succeeding stage until the desired discharge pressure is achieved. Thus, the steam properties--that is, temperature, pressure, velocity and moisture content--vary from row to row as the steam expands through the flow path. At certain locations within the flow path, especially immediately upstream of the last row of rotating blades in a low pressure steam turbine, the steam may be "wet." Under wet steam conditions, water droplets may condense on the stationary vanes adjacent the inner cylinder. Such water droplets may be swept from the inner cylinder into the steam flow and impact the downstream row of rotating blades. Such impact can result in erosion and subsequent weakening of the rotating blades.
In order to improve the thermodynamic performance of the steam turbine exhaust system, an exhaust diffuser is employed. One such exhaust diffuser is shown in U.S. Pat. No. 5,257,906 (Gray et al.). The exhaust diffuser is comprised of inner and outer flow guides. The outer flow guide is typically attached to the blade ring portion of the inner cylinder by means of a bolted vertical flange, although outer flow guides that are integral with the blade ring have also been used. Traditionally, the tips of the last row of rotating blades are enclosed by the flanged area of the outer flow guide. Moisture is typically removed from the steam immediately upstream of the last row of rotating blades by means of a gap formed between the inner cylinder and the flange of the outer flow guide--see, for example, U.S. Pat. Nos. 5,149,248 (Cramer), 4,948,335 (Silvestri), and 3,058,720 (Hart et al.).
Unfortunately, because of the need for small radial clearance between the tips of the rotating blades and the outer flow guide, this approach requires that the outer flow guide be very accurately aligned to the inner cylinder. The need for such careful alignment complicates the manufacture and assembly of the steam turbine.
It has been proposed that a circumferential slot, connected to radial discharge holes, be used for moisture removal in the upstream stages of a steam turbine--see U.S. Pat. No. 3,973,870 (Desai). In this approach, the width of the slot must be fairly large in order to provide sufficient area for the inlets of the discharge holes so as to prevent the accumulation of excessive moisture within the slot. Unfortunately, the swirling of the steam into such a large width slot can result in moisture being entrained into the steam flowing downstream of the blades, thereby defeating the purpose of the moisture removal.
It is therefore desirable to provide a moisture removal system in a steam turbine in which the danger of entrainment of moisture into the steam flow is minimized, as well as to provide a moisture removal system for the last row of rotating blades in a steam turbine that dispenses with the need for a gap between the outer flow guide and the inner cylinder.
The power output of an existing low pressure steam turbine can be increased by a retrofit which includes increasing the length of the last row of rotating blades. If the hub diameter of the new last row of rotating blades is changed, the original inner flow guide must be replaced by one that mates with the new blade row so as to provide a smooth path for the flow of steam. It is therefore desirable to provide a scheme for modifying an existing inner flow guide to match the hub diameter of a new set of last row rotating blades.